Lawyers will today begin legal moves to waive death sentences handed to six prisoners in Texas, after the state's attorney general admitted that racial bias had been a factor in their trials.

The admission represents an embarrassment for the Texas governor and presidential contender, George W Bush, who has claimed that he has never presided over the execution of an innocent person during more than five years in office.

The review of the cases was forced on Texas by a supreme court decision last week, which overturned the death sentence of a convicted murderer because an expert prosecution witness at his trial had testified that the defendant's race could be a factor in deter mining whether he should face execution.

Walter Quijano, a clinical psychologist, had argued that Victor Saldano could be considered dangerous on the grounds that Hispanics were "over-represented" in prisons, and therefore more of a threat to society.

Under Texas law, the question of whether the defendant represents "a continuing threat to society" is one of the criteria for imposing the death penalty.

On discovering that Mr Quijano had provided similar testimony in six other cases, resulting in death sentences, the office of the state's attorney general, John Cornyn, notified the defence lawyers in each case that the state would not oppose their motion to have the sentences overturned.

"The people of Texas want and deserve a system that affords the same fairness to everyone," Mr Cornyn said in a statement.

He added that his staff had examined the records of 218 inmates executed in Texas since 1982 and found that none of the executions were based on "this kind of testimony" by Mr Quijano.

It was not clear over the weekend whether previous executions may have resulted from racially charged testimony.

Defence lawyers and civil rights activists have long pointed to the fact that while black Texans represent only 11% of the state population, they account for more than 40% of inmates on death row.

The Texas record on the death penalty has come in for close scrutiny as Mr Bush's presidential bid gathers speed, at a time of creeping unease about the possibility that innocent prisoners are being executed. Texas leads the nation in executions and critics have argued that its system is weighted towards prosecutors.

Much of the criticism has stressed the often poor quality of defence lawyers who - in the absence of a state public defender's office in Texas - are appointed by local courts and frequently lack experience or basic competence.

One death row inmate, Calvin Burdine, is struggling to overturn his 1983 conviction on the grounds that his court-appointed lawyer slept through much of his two-day trial.

Another, Gary Graham, who faces execution on June 22, was convicted on the strength of a single witness to a 1981 murder. His defence lawyer failed to present the jury with a police ballistics report showing that the lethal shot in the case could not have been fired from Graham's gun. He also failed to interview up to six witnesses who later said Graham was not the man they saw carry out the killing.

In a New York Times interview yesterday, Graham's former lawyer, Ron Mock, admitted he had failed law school exams at a Texas university, had at one time owned 11 bars at the same time as practising law, and been a heavy whisky drinker at the time of the trial, but he claimed: "It never affected my performance."

Five other death row inmates have petitions against Mr Mock for providing ineffective legal defence.